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NATIONAL BUREAU OF STANDARDS REPORT

MECHANICAL EVALUATION OF SOME
HIGH SPEED HANDPIECES
by

Duane F, Taylor
Robert Rs Perkins
John W„ Kumpula

U. S. DEPARTMENT OF COMMERCE
NATIONAL BUREAU OF STANDARDS

6433

Progress Report

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NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT MBS REPORT

0708-20-3824 June 12 y 1959 6433

Progress Report
on

MECHANICAL EVALUATION OF SOME
HIGH SPEED HANDPIECES

Duane F, Taylor*

Robert R, Perkins**

John W„ Kumpula '

* Metallurgists Dental Research Sections National Bureau
of Standards,

** Guest Workers U, S, Navy, Dental Research Sections
National Bureau of Standards,

' Laboratory Mechanics Dental Research Sections National
Bureau of Standards,

This work is a part of the dental research program conduc-
ted at the National Bureau of Standards in cooperation
with the Council on Dental Research of the American Dental
Associations the Army Dental CorpSs the Dental Sciences
Division of the School of Aviation Medicines USAFs the
Navy Dental Corps., and the Veterans Administration,

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uch permission is not needed,
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U. S. DEPARTMENT OF COMMERCE
NATIONAL BUREAU OF STANDARDS

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MECHANICAL EVALUATION OF SOME

HIGH SPEED HANDPIECES

Abstract

The power transmission characteristics
of a series of dental air turbine handpieces were
studied . The torque produced as a function of
speed was determined for each handpiece tested.

The effects of air pressure., instrument size, and
instrument balance upon the speed and power were
investigated. Increased pressure was found to be
more effective in increasing power than in increas-
ing speed. The size of the instrument was found to
have negligible effect upon the power output in com-
parison to the effect of dynamic balance.

I o INTRODUCTION

The introduction in the last few years of a wide
variety of high-speed dental handpieces and instruments
has emphasized the need for some method of evaluating dental
cutting procedures » The dental literature has been filled
with papers related to the use of high speeds, and a
complete bibliography on the subject would run to several
hundred items for the last five years alon@0 Only a
very few of these papers, however, have been concerned
with the mechanism of cutting or the instruments
themselves /”l-3__7o The majority have been directed
toward techniques for their clinical use*

The Dental Research Section of the National Bureau
of Standards has for several years conducted a program
of research into various non-elinical aspects of the
dental cutting problem One portion of this

program has been the investigation of the mechanical
operating characteristics of dental handpieces „ This
paper presents our findings and conclusions in regard
to one group of these handpieces, the air turbines „

The program as a whole is directed toward providing
a more fundamental understanding of the cutting process
with particular emphasis upon the energy considerations
involvedc A wide variety of handpieces was studied to

provide basic information in regard to the conditions
that are encountered clinically. These handpieces included
belt-driven designs having maximum speeds from 6,000 to
150,000 rpm, and water turbines, as well as the air tur-
bines discussed here. It is hoped that this study not
only will provide a basis for quantitative measurement
of the characteristics of different types of handpieces
but also will lead to a better understanding of the cutting
process itself.

The method employed involves measurement of the
energy transferred at several places within the cutting
equipment, most particularly the energy transfer from
the handpiece to the instrument and from the instrument
to the surface being cut. This permits the comparison
of the useful work obtained (cutting) to the losses (heat,
noise and vibration) and allows an evaluation of the
relative efficiency of various handpiece-instrument com-
binations. A detailed description of the equipment de-
veloped and methods employed in this study appears else-
where /TO/7, and only a short discussion will be given here,

II. EXPERIMENTAL PROCEDURE

The air turbines tested are listed in Table 1. As
indicated there, the number of samples tested ranged from

- 3 -

one to four. The Midwest handpiece tested was an
experimental design employing latch-type burs rather than
the friction-grip design presently available. The other
handpieces were all standard commercial models.

The operating characteristics of the handpieces
were studied by determining the torque delivered
to the instrument shaft by the handpiece at various speeds.
An overall view of the equipment used for this purpose
is shown in Figure 1, while a close-up view with a hand-
piece mounted for testing appears in Figure 2.

This apparatus was designed to produce a braking
effect upon the handpiece by interaction between a
synchronous magnetic field and a permanent magnet
ferrite cylinder mounted in the handpiece. The braking
.field was brought into synchronization with the handpiece
and the field strength increased until the handpiece
speed was controlled by the field frequency. The hand-
piece speed was then varied by changing the frequency
while the torque produced at various speeds was observed.

Calibration and reading errors cause a relative
uncertainty of about five percent between the individual
observations on a single run for both speed and torque.
Absolute values of torque are known to the same accuracy

- 4 -

but drift in calibration of the frequency measuring system
induces a maximum additional uncertainty of 5$ in the
absolute speed determinations.

Results and Discussion

A typical torque-speed relationship found by this
technique is shown in Figure 3° The shape of this curve
is typical of all of the turbines tested. It has been
generally observed clinically that the torque produced by
the air turbines is much smaller than that produced by
conventional belt-driven handpieces. For example, the
maximum torque produced in this run, about 10 gram-
centimeters, is attained at minimum speed and may be
compared with the 250-300 gram-centimeters maximum torques
achieved by typical ball bearing belt-driven handpieces
as shown in Figure 4.

The results obtained, particularly in some of the
early runs, were not always as regular as those of Figure 3*
Data of the type shown in Figure 5 were often obtained and
in some cases the curves were as irregular as that shown
in Figure 6. In this case, at the air pressure used,

(30 psi), the handpiece would reach a maximum speed of
only 110,000 rpm. When that speed was approached the
handpiece became increasingly noisy and the cylinder ran

- 5 -

very eccentrically. Gradually increasing the air pressure
made little difference in the speed until near 50 psi,
when the speed suddenly increased to 400,000 rpm. Re-
ducing the pressure to 30 psi produced a free running
speed of 360,000 rpm. From that point the torque-
speed curve could be traced to 300,000 rpm where the
torque dropped toward zero. It was impossible to follow
the curve continuously through this range and the middle
section (between 300,000 and 100,000 rpm) was derived only
by pressure cycling. When this was done, however, it
was possible to follow this portion of the curve satis-
factorily at 30 psi pressure.

This behavior was attributed to a resonance effect.
The regular spacing of the speeds at which reduced torques
appear, support this conclusion. Major drops occurred
near 100,000 and 300,000 rpm and a smaller drop at
200,000 rpm. Although the magnet cylinders and the
handpiece rotors were themselves well balanced, it was
conceivable that, when the magnets were inserted in the
handpiece, errors of alignment or centering might make
the rotor-cylinder combination dynamically unbalanced.

As a result, an attempt was made to grind the magnet
cylinder into dynamic balance while driving them with the
handpiece.

- 6 -

Figure 7 shows three of the magnet cylinders used.

The one on the left has been ground and balanced relative
to its shaft, while the others have subsequently been
balanced in a handpiece. Of the three, the one on the
right ran the fastest in spite of its rough appearance.
Improvement, when it occurred, was often dramatic though
not readily predictable. Table 2 shows the results
of a series of successive grinding passes on three
cylinders being balanced for the same handpiece.

The particular rotor with which these data were
obtained was very sensitive to defects of balancing,
although a previous rotor in the same handpiece had
shown a much less marked effect.

Somewhat similar, though less extreme behavior was
observed with burs. When several burs of the same size were
tested, they were commonly observed to run at different
maximum speeds. Alsq, reducing the pressure to the
handpiece to levels of 5 to 10 psi at times produced
results with burs which closely resembled those with
unbalanced cylinders. In the case of the burs, however,
increase of pressure back to the normal operating range or
removing and repositioning the bur in the handpiece were
sufficient to let the handpiece escape the low speed

- 7 -

resonance and achieve normal free-running speeds.

Apparently the same type of effects occur with the
cylinders and with hurs. However, because of the
relatively small mass of the burs the effect becomes
apparent only under those circumstances where the torque
of the handpiece is already marginal, near maximum speed
or at very low pressures. Diamond instruments should be
expected to produce similar results intermediate to
these two cases.

Table 3 illustrates the importance of this factor in
determining handpiece speeds. It compares the maximum
speeds attained with three magnet cylinders and four burs
at a series of pressures. The cylinders were all balanced
by grinding in the handpiece used for the tests, a Weber
AT 200. Note that of the three cylinders, .No. 1 which
was the slowest running of the three still shows signs
of resonance problems and is apparently not fully balanced.
It shows a one-third increase in speed between 20 and
30 psi from 180,000 to 240,000 rpm, (both multiples of
60,000), and not until the pressure was increased to
60 psi was any further increase in speed obtained. The
other cylinders and the burs showed a continual increase
in speed with pressure.

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Probably the most significant point relative to this
table, however, is that, in spite of their large size
(approx.1 .240 x .400 inch) and the irregular surface
produced in the balancing operation, cylinders 2 and 3 ran
as fast or faster than any of the burs. This indicates
that size alone is no detriment for high speed operation
and that air drag must play a very minor role in controlling
speed. It also appears that an eccentric bur or worn
chuck that unbalances the rotor can result in considerable
slowing of the handpiece and reduced performance.

Typical torque-speed cpr>eg for the various handpieces
tested with resonance effects eliminated are shown in
Figure 8. All curves are for 30 psi air pressure which
is the approximate maximum pressure allowed by the
regulators in the S. S. White and Ritter handpieces tested.

Of the six curves shown, numbers 1, 2, and 6 are
for handpieces using latch-type contra-angle burs while
the other three handpieces use friction-grip burs. The use
of the larger shank, latch-type bur results in a larger
overall head size for the handpiece and appears to
require a greater air supply. Of the three latch- type
handpieces tested, only one, the Weber AT 200 is marketed
at the present time. It has replaced the Weber model 700.

_ 9 -

The Midwest handpiece studied has been replaced by a
friction-grip handpiece which was not tested.

In those cases where several samples of a single
handpiece design were tested, considerable differences
in performance were noted. In addition, smaller changes
occurred from day to day and run to run with a single
handpiece. These variations together amount to as much as
± 15$ from the mean and appear to be due mainly to such
causes as irregular lubrication and bearing wear.

While the torque-speed curve provides the basic
information in regard to the energy transferred from the
handpiece to the bur, in many cases, it is more informative
as well as more convenient to work with power-speed data.
Since the power of a rotating device is equal to the
product of the speed and torque, the power-speed curve
is readily derived from the torque-speed measurements.

The power curves corresponding to the runs of Figure 8

are shown in Figure 9° For comparison the belt-driven

handpiece of Figure 4 develops a maximum power of 25.9
watts at 19 9 000 rprru

The effect of variations in pressure upon the per-
formance of one air turbine handpiece is shown in Figures
10 and 11. The values given are for the Weber AT 200,

10 -

which is shown here because of the range of pressures
usable with this handpiece. A wide pressure range is
definitely advantageous for laboratory investigation since
it permits the study of a variety of speeds and powers
with a single handpiece.

The clinical necessity for very high pressures is
doubtful and some makes of handpieces have regulators
which permit a maximum pressure of 30 psi.

The curves of Figures 10 and 11 show certain
characteristics common to all of the air turbines tested.
The effect of increasing air pressure is much greater
upon the torque than upon the speed. As seen in Figure
10, doubling the air pressure will approximately double
the maximum torque developed but will increase the
maximum speed only 1 6$. The influence of design appears
to be much more important than air pressure in determining
maximum handpiece speed. However, the pressure used may
make a considerable difference in the speed reached
during cutting. Figure 10 indicates that if the handpiece
is required to supply five gram-centimeters of torque to
the instrument during cutting, (a high value for this
speed range), the handpiece will run 270,000 rpm at 60 psi,
240,000 rpm at 50 psi, 205,000 rpm at 40 psi, 135,000 rpm

at 30 psl, and will stall completely at 20 psi.

Figure 11 shows that the maximum power available
increases rapidly with increased pressure. The speed
at which the maximum power occurs is also seen to
increase with pressure.

Unfortunately, from the clinical point of view,
information is not yet available as to how much power
is needed or desirable for operative use. It is now
possible to make a start in that direction by making
cutting tests using handpieces whose operating charac-
teristics are known. Provided with the data of Figure 11,
it is possible, for example, to make a series of cuts
at a constant speed with varying powers (shown by the
dots in Figure 11 ) or at constant power with varying
speeds (shown by crosses). Study of the cutting performed
and heat produced under these conditions should permit
the determination of the effect of both factors upon
cutting efficiency. Such a program has been undertaken
in the expectation that it will eventually lead to a
better understanding of the dental cutting process as

a whole

III. SUMMARY

The operating characteristics of several designs
of air turbine handpieces have been investigated. Their
ability to transmit energy to the shaft of the cutting
instrument was studied by means of an electro -dynamic
brake, and was found to be strongly dependent upon the
dynamic balance of the instrument and upon the air
pressure used. This method provides a means for the
comparison of various air turbine handpieces and yields
data which can be used as a basis for the study of the
mechanism and efficiency of dental cutting procedures.

Bibliography

1. Peyton, Floyd A., Evaluation of Dental Handpieces

for High Speed Operations. J.A.D.A. 50; 383-391 *

April 1955„

2. Bernier, J. L. and Knapp, M. J„, Methods Used in
Evaluation of High Speed Dental Instruments and
Some Results. Oral Surg., Oral Med. and

Oral Path. 12; 234-252, February 1959.

3. Rigas, D. J., Skinner, E. W., Lindenmeyer, R. S.,
and Lasater, R. L., Design Factors of Dental
Burs as Related to Cutting Effectiveness,

J. D. Res. 37: 91 * February 1958, Abstract.

4. Nelsen, R. J., Pelander, C. E. and Kumpula, J. W. ,
Hydraulic Turbine Com tr3. -angle Handpiece, J.A.D.A.,
47: 324, September 1953.

5. Hudson, D. C., and Sweeney, W. T. , Temperatures
Developed in Rotating Dental Cutting Instruments,
J.A.D A. 48; 127, February 1954.

6. Hudson, D. C., Hartley, J. L., Moore, Robert and
Sweeney, W„ T., Factors influencing the Cutting
Characteristics of Rotating Dental Instruments,
J.A.D.A. 50: 377, April 1955.

7. Dental Burs in Action. Film available on loan from
the Office of Scientific Publications, National
Bureau of Standards, Washington 25, D. C., or the
American Dental Association Film Library, 222

East Superior St., Chicago 11, Illinois.

8. Hartley, J. L., Hudson, D. C., Sweeney, W. T. , and
Richardson, W. P., Cutting Characteristics of Dental
Burs. Armed Forces Medical J. 8; 209, February 1957.

9. Hartley, J. L., Hudson, D. C., Sweeney, W. T. , and
Dickson, George., Methods for the Evaluation of
Rotating Diamond-abrasive Dental Instruments.,

J. A. D. A., 54: 637, May 1957.

10. Perkins, Robert R., Taylor, Duane F., and Kumpula,

J. W., Evaluation of Dental Cutting Procedure;

Method and Apparatus (in preparation).

Table 1

Air Turbine Handpieces Tested

Handpiece

Humber of Samples

Densco

Aero Turbex

Midwest

Air Drive

Ritter

Air Rotor

S.S. White

Air Rotor

Weber *

Air Turbine

700

Weber

Air Turbine

AT 200

Table 2

The Effect of Balancing Upon Maximum Rotational Speed

Magnet Cylinder

Initial Speed

80,000 rpm

50,000 rpm

1st Grinding

85,000

75,000

2nd Grinding

85,000

280,000

^rd Grinding

320,000

70.000 rpm

220,000

50.000

210,000

Handpiece

Pressure

Cylinder

Weber AT- 200 Air Turbine

30 psl

.240 Diam. x .400 length approx.

The Effect of Air Pressure Upon the Rotational Speed of Various Instruments

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Figure 1.

Apparatus employed for the high speed -
low torque measurements.

SPEED 1000 rpm

Figure 3. Torque - speed curve typical of air turbine handpieces e

SPEED iooo

Figure 5. Torque - speed curve showing mild resonance effects.

Figure 6. Torque - speed curve showing extreme resonance effects.

Figure 7

I INCH

View showing the effect of grinding for dynamic balancing
upon the shape of magnet cylinders.

■

400 -

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350

300 -

250

200 -

150 -

100 -

POWER-SPEED CURVES
AIR TURBINE HANDPIECES

MIDWEST

2 WEBER A T 2 0 0

3 SS WHITE

4 RITTER

5 DENSCO

6 WEBER 700

POWER watt s

Figure 9. Power - speed curves for various air turbine handpieces^
derived from the data of Figure 8.

Figure 10. Torque - speed curvefj showing the effect of variation in air

pressure .

SPEED iooo

Figure 11. Power - speed curves showing the effect of variation in air
pressure .

u. S. DEPARTMENT OK COMMERCE

Lewis lj. Strauss, Srrrflnty

national mmtcAu OF STANDARDS
A. V. AaOiI, Director

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Full text of "Mechanical evaluation of some high speed handpieces"

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